This invention generally relates to an ink-jet recording method and an ink-jet recording apparatus using the same. More specifically, the invention relates to an ink-jet recording method for recording an image on a recording sheet by generating a Rayleigh surface acoustic wave (SAW), providing an ink material on a propagation path of the SAW, jetting a droplet of the ink material by the SAW and adhering the ink droplet onto the recording paper.
Several on-demand type ink-jet recording apparatuses for recording an image by jetting ink droplets from plural orifices have been proposed. Typical on-demand type ink-jet recording apparatuses are known as the piezoelectric transducer-type ink-jet recording apparatus or the thermal ink-jet apparatus.
In the piezoelectric transducer-type ink-jet recording apparatus, an image is formed by changing the inner pressure of ink material loaded in an ink reservoir by deforming a piezoelectric element located in the ink reservoir, jetting ink droplets from a nozzle connected to the ink reservoir and forming imaging dots on the recording paper. In the thermal ink-jet recording apparatus, an image is formed by producing a bubble in an ink material loaded into an ink reservoir by heating the ink material by a heating element located within the ink reservoir, jetting ink droplets from a nozzle connected to the ink reservoir by the generating pressure of the bubble and forming imaging dots on the recording paper.
By using those ink-jet recording methods, typically an image having 300 dot per inch (dpi) image resolution or higher image resolution such as 600 dpi or 720 dpi may be obtained. However, much higher image resolution for the ink-jet recording apparatus has also been required.
It is necessary to reduce the diameter of ink droplets to achieve such high resolution. Typically, the nozzle diameter is set to a much smaller diameter to make the recording dot small, however, the small nozzle diameter also induces inner-nozzle clogging problems due to foreign matters or dried ink material residing in the nozzle, or the small nozzle diameter may also cause a direction changing problem of the jetting ink due to adhering of the ink residue around the ink nozzle. As a result, image defects on the recording paper may occur.
It is not always a good way to reduce the nozzle diameter to reduce the recording dot because there is a certain lower limit of the nozzle diameter ensuring both such higher resolution and quality of the recorded image.
Recently, a new ink-jet recording method, different from the aforementioned piezoelectric-type or the thermal-type ink-jet recording, utilizing a surface acoustic wave (SAW) has been proposed. A surface acoustic wave, which transports all of its energy within a depth of one wavelength thereof from the surface of a solid wave transporting medium, is generally referred to as a Rayleigh surface acoustic wave. If liquid is existing on the surface of the wave transporting medium, the Rayleigh surface acoustic wave leaks out into the liquid and is attenuated at the solid surface thereof. Thus, the Rayleigh surface acoustic wave is released at the liquid surface as an energy form of liquid supersonic wave while the Rayleigh surface acoustic wave is still transporting in the transporting medium. The liquid supersonic wave is generally referred to as a leaked Rayleigh surface acoustic wave. At this time, a longitudinal wave is released into the liquid with a certain angle direction. By using such phenomenon for transporting the energy as a specific wave form, ink droplets will be produced in the liquid and are jetted onto the recording paper.
According to this feature, as the ink jetting opening does not have to be a form of orifice or nozzle or should not have the same diameter as the ink droplet, the diameter of the ink droplet is not affected by the diameter of the ink nozzle. Also, the ink jetting nozzle does not have to be formed as a shape of circular nozzle but may be formed as a slit. The shape of the ink jetting opening will not be a critical matter on this system.
Ink-jet recording system utilizing the Rayleigh surface acoustic wave is disclosed in Japanese unexamined patent publications (JP-A) 54-10731 (1979) and 62-66943 (1987). In those systems, surface acoustic waves are generated by interdigital electrodes positioned within the ink liquid and the liquid ink is vibrated by the leaked Rayleigh surface acoustic waves to form ink droplets to be discharged from the ink jetting opening like a nozzle.
In this system, deterioration of the ink material or the interdigital electrodes tend to occur because the interdigital electrodes directly contact the ink material so that the electrodes and the ink material react with each other to melt the electrode material or to adhere ink residues onto the electrodes. Also the energy transporting efficiency is relatively low because the longitudinal wave generated by the leaked Rayleigh surface acoustic wave on the solid transporting medium tend to be easily reduced within the ink liquid.
To avoid such problems, other ink-jet recording systems having interdigital electrodes that do not directly contact the ink material are proposed and disclosed in the examined patent publications (JP-A) 2-269058 (1990) and 4-14455 (1992).
FIG. 19 is an explanatory view of the principal structure of the conventional ink-jet recording method utilizing the surface acoustic wave. In FIG. 19, 31 is the surface, 32 are the interdigital electrodes, 33 is the piezoelectric plate, 34 is the ink, 35 is the ink droplet, and 36 is the high frequency power source. The interdigital electrodes are formed onto the surface 31 of the piezoelectric plate 33. When the high frequency voltage from the power source 36 is applied to the interdigital electrodes 32, surface acoustic waves are generated and are transported through the surface 31 of the piezoelectric plate 33. Once the ink 34 is placed onto a propagating path of those surface acoustic waves, the vibrating energy of the surface acoustic wave will be transferred onto the ink 34 to produce an ink droplet 35 to be ejected.
There is no ink supplying mechanism in this principal FIG. 19. Therefore there is no way to refill the ink 34 after the ink has been fully consumed so as to continuously produce ink droplets for recording. JP-A 2-269058 discloses a capillary for the purpose of the refilling of the ink. JP-A 4-14455 discloses a slit for providing the ink material continuously on the propagating path of the piezoelectric element.
The systems disclosed in JP-A 2-269058 and JP-A 4-14455 have relatively high reliabilities because the ink does not directly contact the interdigital electrodes, and the ink jetting opening does not have to be formed as a small orifice as small as the produced ink droplet. In addition, generated energy of the surface acoustic wave will be efficiently used as the ink jetting energy because attenuation of the surface acoustic wave will be maintained minimal until the waves contact the ink material, and an actual transporting distance of the leaked longitudinal wave within the ink will also be very short.
However, ink-jet systems utilizing such leaked Rayleigh surface acoustic wave still have a problem that ink discharging condition will be changed based on the positional relationship between the propagating path and the provided ink material.
The characteristic of the leaked Rayleigh acoustic waves as being a longitudinal wave leaked from a solid surface that propagates the surface acoustic wave, as well as the jetting phenomenon of the produced ink droplets has been reported in detail for example in DENSHI-TSUSHIN GAKKAI GIJYUTSU HOKOKU US89-51, pp41-46. According to the analysis of the author of this article, the angle of the longitudinal wave leaked from the solid surface material or Inter-Digital Transducer (IDT) on the surface of the solid surface is estimated as the following equation: EQU Leaked Rayleigh angle .alpha.=sin.sup.-1 (Vi/Vw)
Wherein, Vw is the velocity of the leaked surface acoustic wave on the solid surface which contacts to the liquid contacting to liquid, and Vi is the velocity of the longitudinal wave transporting within the liquid. This angle is primarily estimated as, for example .alpha. is 23.degree. in the system using water as the liquid and the LiNbO.sub.3 having 28.degree. Y-plate-X transporting as the solid. This value was also confirmed by actual experiment. The phenomenon of the leaking of the longitudinal wave into the liquid from the solid surface occurs at the same time when the surface acoustic wave propagating on the solid surface contacts the liquid. The surface acoustic wave (leaked surface acoustic wave or leaked Rayleigh wave) on the surface of the solid will be attenuated within a length equal to a few wavelengths in the liquid when the surface is immersed into the liquid. Therefore, a producing position of the ink droplet and a jetting direction of the generated ink droplet will be easily affected by the actual contacting position between the liquid and the solid surface, which also affects the generating position of the longitudinal wave that will be transported in the liquid. Therefore, it is impossible to produce dots accurately on the recording paper due to the inaccuracy of the jetting position of the ink droplets unless the amount of the providing ink and the providing position of the ink is accurately controlled in the system of JP-A 2-269068 or surface position is accurately controlled of the liquid ink in the system of JP-A 4-14455.
In addition, it is necessary to shorten the width d of the interdigital electrodes 32 to reduce the diameter of the produced ink droplets in the conventional system disclosed in FIG. 19. However, if the width d is decreased to be not more than 1/10 width of the wavelength .lambda. of the surface acoustic wave to be generated, a directivity of the transporting direction of the surface acoustic wave will be much worse. Also, the worse directivity enhances the occurrence of unnecessary vibration sufficient to produce cross-talk on the liquid surface and instability of the jetting direction of the droplets.
To produce relatively small droplets without such problems, it is plausible to maintain the width d to be not less than 10 times of the wavelength .lambda. of the surface acoustic wave by shortening the wavelength .lambda. of the surface acoustic wave (to generate a relatively high wave) and by narrowing the width d. However, the oscillating frequency will be inaccurately high sufficient to produce alternate drawback such as a requirement of relatively expensive high frequency power source. This is not a critical solution of the aforementioned problems. Thus, it is still difficult to accurately produce relatively small ink droplets and to jet the droplets onto the recording sheet.